Currently known DC/DC converters are of various types, including buck, boost or buck/boost types. The buck and boost types develop decreased and increased output voltages relative to an input voltage, respectively. The buck/boost converter can function as either a buck or a boost converter depending upon the duty cycle of one or more switches in the converter.
Most DC/DC converters utilize power switches which control the flow of power through a transformer primary winding. An induced voltage established in a transformer secondary winding is rectified and filtered to produce the DC output. Such types of converters are necessarily large and heavy due to the use of the transformer.
A further type of converter is disclosed in Paulkovich et al U.S. Pat. No. 4,245,286. This patent discloses a buck/boost regulator which includes a first switch which is coupled between an input of the regulator and an inductor and a transformer winding. A capacitor and load are coupled through diodes to the inductor and the transformer winding. A pulse width modulator senses the voltage across the load and controls the first switch as well as second and third switches which are coupled across the ends of the transformer winding to control the voltage delivered to the load.
The Paulkovich regulator, however, is relatively complex in that it requires a large number of components together with a transformer winding.
There may be times when it is necessary or desirable to transfer power bi-directionally through a DC/DC converter. For example, as disclosed in Dishner et al U.S. patent application Ser. No. 893,943, filed Aug. 6, 1986, entitled "Power Converter for an Electrically-Compensated Constant Speed Drive" and assigned to the assignee of the instant application, the disclosure of which is hereby incorporated by reference, there is disclosed an electrically-compensated constant speed drive which develops constant speed motive power from variable speed motive power provided by a prime mover. The drive includes first and second permanent magnet machines having electrical power windings which are interconnected by first and second AC/DC converters and a DC/DC converter. In this constant speed drive, one of the machines is operated as a generator while the other is operated as a motor so that the speed of an output shaft of the drive is maintained constant even with changes in speed of the prime mover. The DC/DC converter used in this constant speed drive is complex in nature since it must be capable of bi-directional operation, i.e. it must be capable of transferring power from one machine to the other and vice versa, and it must be capable of boosting or bucking the voltage in both directions of power flow. The converter includes steering switches which connect an input of the converter to one of the AC/DC converters and an output of the converter to the other of the AC/DC converters, an inverter for converting DC power developed by one of the AC/DC converters into intermediate AC power, a transformer for shifting the voltage of the intermediate AC power and a rectifier for rectifying the output of the transformer so that output DC voltage is developed. Bypass switches are also provided in the event it is desired to bypass the DC/DC converter.
While the DC/DC converter described in the above-referenced patent application performs satisfactorily for its intended purpose, it would be desirable to reduce the number of components used therein to in turn reduce the size and weight of the overall constant speed drive.
It has been found that by appropriately limiting the speed range of the prime mover, the DC/DC converter can be simplified since the converter need only buck voltage when power flow is in one direction and need only boost voltage when power flow is in the opposite direction. A converter which is capable of such operation is disclosed in Dishner U.S. patent application Ser. No. 946,086, filed Dec. 23, 1986, entitled "Boost/Buck DC/DC Converter" and assigned to the assignee of the instant application.